(CNN) -- In a mystery straddling two states, three men and a woman were found dead of shotgun blasts in north Alabama after two of them were targeted in a Tennessee child pornography investigation, police say.

It's being called a triple murder-suicide pact, as it appears the shooter killed his sister, former brother-in-law and friend before turning the 12-gauge on himself.

It's unclear why they were in Alabama and why two of the four chose to take part in the pact.

The four left handwritten suicide notes claiming their innocence and making requests for their funeral arrangements. Authorities discovered the notes, along with the bodies, in a car near Double Springs, Alabama, last week, Sheriff Rick Harris in Winston County, Alabama, told CNN.

The car, parked on a logging road in a national forest, had its engine running, windshield wipers on and radio blaring when deputies located it, according to CNN affiliate WATN in Memphis.

Kristie Hunt Campbell Hamrick, 39; her ex-husband, Robert Samuel Hamrick, 30; her brother, Andy Hunt, 38; and family friend Kevin Carey, 30, who all listed the same Savannah, Tennessee, address as their home, were inside.

In an interview with CNN affiliate WBRC in Birmingham, Harris said it appears Hunt fired five shots from a Mossberg 500A pump shotgun, killing the other three before getting into the car and shooting himself.

"We think at this point, by the shot pattern and trajectory study they did, that he walked around the car, and literally just leaned into the car and did what he had to do," Harris told WBRC.

Campbell Hamrick and her ex-husband became targets of a pornography probe after a teenage girl made allegations in September to the Department of Children's Services, which contacted the Hardin County, Tennessee, police, said Sheriff Samuel Davidson.

Deputies searched the four's mobile home the next day and seized three computer towers, two flash drives, two cameras and two cell phones, Davidson said. A forensics lab is expected to complete its analysis of the seized items next month.

Next-door neighbor Danny Keymon told WATN he saw police last month take "a bunch of junk out of here." His neighbors later vanished, and Robert Hamrick's mother told Keymon that they'd been involved in a devastating truck accident, he said.

"We heard they had a bad 18-wheeler wreck out in rural Texas and cut both of Kristi's legs off and one of Andy's arms and one of his legs and killed Kevin and threw Robert through the windshield," Keymon said.

Pam Dickerson, another next-door neighbor, told the station that though she'd had confrontations with Robert Hamrick in the past, he didn't seem like the type to be involved in child pornography.

"I've taken him home from town, too, when he was walking. He didn't seem like the type who could do that," she told WATN.

No charges have been filed in the child pornography probe, and Hunt and Carey were not being investigated, Sheriff Davidson said.

When police went October 3 to the Savannah trailer where the four lived, the person who answered the door told police they'd been missing for two weeks. Authorities in Alabama had discovered them the night before, Davidson said.

Hardin said there will be no charges in relation to the murder-suicide, though the investigation remains open as authorities try to determine what exactly happened and why the foursome chose the Black Pond community in the William B. Bankhead National Forest to take their lives.

So far, investigators know that Carey had relatives in Alabama with whom he had discussed the suicide pact. The relatives told investigators of the conversation after Carey's death, Harris told CNN.

Harris also told WBRC that Carey told a friend in Hamilton, Alabama, that he and the other three planned to kill themselves rather than go to prison on child pornography charges.